I'm glad to hear the recipient has changed her attitude. That will make life easier for you. I never had that situation. In fact, my friend who received my kidney was gracious beyond words, to the point it was almost embarrassing to me. I didn't know if saying, "You're welcome" was sufficient. I know how grateful he is for what I did, and deep down, no matter what, that's probably how your sister-in-law is thinking. She probably didn't have the right words to express her gratitude. Never having been a recipient, I don't know exactly how I would react. When I look at it from the recipient's point of view, it does seem like thanking someone for donating is beyond words.
Best wishes.